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<h1>C++ <span class="color_h1">基本语法</span></h1>

<div class="tutintro">
<p>C++ 程序可以定义为对象的集合，这些对象通过调用彼此的方法进行交互。现在让我们简要地看一下什么是类、对象，方法、即时变量。</p>
<ul class="list">
<li><b>对象 -</b> 对象具有状态和行为。例如：一只狗的状态 - 颜色、名称、品种，行为 - 摇动、叫唤、吃。对象是类的实例。</li>
<li><b>类 -</b> 类可以定义为描述对象行为/状态的模板/蓝图。</li>
<li><b>方法 -</b> 从基本上说，一个方法表示一种行为。一个类可以包含多个方法。可以在方法中写入逻辑、操作数据以及执行所有的动作。</li>
<li><b>即时变量 -</b> 每个对象都有其独特的即时变量。对象的状态是由这些即时变量的值创建的。</li>
</ul>
</div>

<h2 class="tutheader">C++ 程序结构</h2>
<p>让我们看一段简单的代码，可以输出单词 <i>Hello World</i>。</p>
<div class="example">
<h2 class="example">实例</h2>
<div class="example_code">
<div class="hl-main"><span class="hl-prepro">#include</span><span class="hl-prepro"> </span><span class="hl-quotes">&lt;</span><span class="hl-string">iostream</span><span class="hl-quotes">&gt;</span><span class="hl-prepro"></span><span class="hl-code">
</span><span class="hl-reserved">using</span><span class="hl-code"> </span><span class="hl-types">namespace</span><span class="hl-code"> </span><span class="hl-identifier">std</span><span class="hl-code">;
 
</span><span class="hl-comment">// main() 是程序开始执行的地方</span><span class="hl-comment"></span><span class="hl-code">
 
</span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">main</span><span class="hl-brackets">(</span><span class="hl-brackets">)</span><span class="hl-code">
</span><span class="hl-brackets">{</span><span class="hl-code">
   </span><span class="hl-identifier">cout</span><span class="hl-code"> &lt;&lt; </span><span class="hl-quotes">&quot;</span><span class="hl-string">Hello World</span><span class="hl-quotes">&quot;</span><span class="hl-code">; </span><span class="hl-comment">// 输出 Hello World</span><span class="hl-comment"></span><span class="hl-code">
   </span><span class="hl-reserved">return</span><span class="hl-code"> </span><span class="hl-number">0</span><span class="hl-code">;
</span><span class="hl-brackets">}</span></div>
</div></div>
<p>接下来我们讲解一下上面这段程序：</p>
<ul class="list">
<li>C++ 语言定义了一些头文件，这些头文件包含了程序中必需的或有用的信息。上面这段程序中，包含了头文件 <b>&lt;iostream&gt;</b>。</li>
<li>下一行 <b>using namespace std;</b> 告诉编译器使用 std 命名空间。命名空间是 C++ 中一个相对新的概念。</li>
<li>下一行 <b>// main() 是程序开始执行的地方</b> 是一个单行注释。单行注释以 // 开头，在行末结束。</li>
<li>下一行 <b>int main()</b> 是主函数，程序从这里开始执行。</li>
<li>下一行 <b>cout &lt;&lt; "Hello World";</b> 会在屏幕上显示消息 "Hello World"。</li>
<li>下一行 <b>return 0;</b> 终止 main( )函数，并向调用进程返回值 0。</li>
</ul>

<h2 class="tutheader">编译 &amp; 执行 C++ 程序</h2>
<p>接下来让我们看看如何把源代码保存在一个文件中，以及如何编译并运行它。下面是简单的步骤：</p>
<ul class="list">
<li>打开一个文本编辑器，添加上述代码。</li>
<li>保存文件为 hello.cpp。</li>
<li>打开命令提示符，进入到保存文件所在的目录。</li>
<li>键入 'g++ hello.cpp '，输入回车，编译代码。如果代码中没有错误，命令提示符会跳到下一行，并生成 a.out 可执行文件。</li>
<li>现在，键入 ' a.out' 来运行程序。</li>	
<li>您可以看到屏幕上显示 ' Hello World '。</li>
</ul>
<pre>
$ g++ hello.cpp
$ ./a.out
Hello World
</pre>
<p>请确保您的路径中已包含 g++ 编译器，并确保在包含源文件 hello.cpp 的目录中运行它。</p>
<p>您也可以使用 makefile 来编译 C/C++ 程序。</p>

<h2 class="tutheader">C++ 中的分号 &amp; 语句块</h2>
<p>在 C++ 中，分号是语句结束符。也就是说，每个语句必须以分号结束。它表明一个逻辑实体的结束。</p>
<p>例如，下面是三个不同的语句：</p>
<div class="example"> 
<div class="example_code">
<div class="hl-main"><span class="hl-identifier">x</span><span class="hl-code"> = </span><span class="hl-identifier">y</span><span class="hl-code">;
</span><span class="hl-identifier">y</span><span class="hl-code"> = </span><span class="hl-identifier">y</span><span class="hl-code">+</span><span class="hl-number">1</span><span class="hl-code">;
</span><span class="hl-identifier">add</span><span class="hl-brackets">(</span><span class="hl-identifier">x</span><span class="hl-code">, </span><span class="hl-identifier">y</span><span class="hl-brackets">)</span><span class="hl-code">;</span></div>
</div>
</div>
<p>语句块是一组使用大括号括起来的按逻辑连接的语句。例如：</p>
<div class="example"> 
<div class="example_code">
<div class="hl-main"><span class="hl-brackets">{</span><span class="hl-code">
   </span><span class="hl-identifier">cout</span><span class="hl-code"> &lt;&lt; </span><span class="hl-quotes">&quot;</span><span class="hl-string">Hello World</span><span class="hl-quotes">&quot;</span><span class="hl-code">; </span><span class="hl-comment">// 输出 Hello World</span><span class="hl-comment"></span><span class="hl-code">
   </span><span class="hl-reserved">return</span><span class="hl-code"> </span><span class="hl-number">0</span><span class="hl-code">;
</span><span class="hl-brackets">}</span></div>
</div>
</div>
<p>C++ 不以行末作为结束符的标识，因此，您可以在一行上放置多个语句。例如：</p>
<div class="example"> 
<div class="example_code">
<div class="hl-main"><span class="hl-identifier">x</span><span class="hl-code"> = </span><span class="hl-identifier">y</span><span class="hl-code">;
</span><span class="hl-identifier">y</span><span class="hl-code"> = </span><span class="hl-identifier">y</span><span class="hl-code">+</span><span class="hl-number">1</span><span class="hl-code">;
</span><span class="hl-identifier">add</span><span class="hl-brackets">(</span><span class="hl-identifier">x</span><span class="hl-code">, </span><span class="hl-identifier">y</span><span class="hl-brackets">)</span><span class="hl-code">;</span></div>
</div>
</div>
<p>等同于</p>
<div class="example"> 
<div class="example_code">
<div class="hl-main"><span class="hl-identifier">x</span><span class="hl-code"> = </span><span class="hl-identifier">y</span><span class="hl-code">; </span><span class="hl-identifier">y</span><span class="hl-code"> = </span><span class="hl-identifier">y</span><span class="hl-code">+</span><span class="hl-number">1</span><span class="hl-code">; </span><span class="hl-identifier">add</span><span class="hl-brackets">(</span><span class="hl-identifier">x</span><span class="hl-code">, </span><span class="hl-identifier">y</span><span class="hl-brackets">)</span><span class="hl-code">;</span></div>
</div>
</div>

<h2 class="tutheader">C++ 标识符</h2>
<p>C++ 标识符是用来标识变量、函数、类、模块，或任何其他用户自定义项目的名称。一个标识符以字母 A-Z 或 a-z 或下划线 _ 开始，后跟零个或多个字母、下划线和数字（0-9）。</p>
<p>C++ 标识符内不允许出现标点字符，比如 @、& 和 %。C++ 是区分大小写的编程语言。因此，在 C++ 中，<b>Manpower</b> 和 <b>manpower</b> 是两个不同的标识符。</p>
<p>下面列出几个有效的标识符：</p>
<pre>
mohd       zara    abc   move_name  a_123
myname50   _temp   j     a23b9      retVal
</pre>

<h2 class="tutheader">C++ 关键字</h2>
<p>下表列出了 C++ 中的保留字。这些保留字不能作为常量名、变量名或其他标识符名称。</p>
<table class="reference notranslate">
<tr><td>asm</td><td>else</td><td>new</td><td>this</td></tr>
<tr><td>auto</td><td>enum</td><td>operator</td><td>throw</td></tr>
<tr><td>bool</td><td>explicit</td><td>private</td><td>true</td></tr>
<tr><td>break</td><td>export</td><td>protected</td><td>try</td></tr>
<tr><td>case</td><td>extern</td><td>public</td><td>typedef</td></tr>
<tr><td>catch</td><td>false</td><td>register</td><td>typeid</td></tr>
<tr><td>char</td><td>float</td><td>reinterpret_cast</td><td>typename</td></tr>
<tr><td>class</td><td>for</td><td>return</td><td>union</td></tr>
<tr><td>const</td><td>friend</td><td>short</td><td>unsigned</td></tr>
<tr><td>const_cast</td><td>goto</td><td>signed</td><td>using</td></tr>
<tr><td>continue</td><td>if</td><td>sizeof</td><td>virtual</td></tr>
<tr><td>default</td><td>inline</td><td>static</td><td>void</td></tr>
<tr><td>delete</td><td>int</td><td>static_cast</td><td>volatile</td></tr>
<tr><td>do</td><td>long</td><td>struct</td><td>wchar_t</td></tr>
<tr><td>double</td><td>mutable</td><td>switch</td><td>while</td></tr>
<tr><td>dynamic_cast</td><td>namespace</td><td>template</td><td>&nbsp;</td></tr>
</table>
<p>完整关键字介绍可查阅：<a href="../w3cnote/cpp-keyword-intro.html" rel="noopener" target="_blank">C++ 的关键字（保留字）完整介绍</a></p>
<br />

<h2 class="tutheader">三字符组</h2>
<p>三字符组就是用于表示另一个字符的三个字符序列，又称为三字符序列。三字符序列总是以两个问号开头。</p>
<p>三字符序列不太常见，但 C++ 标准允许把某些字符指定为三字符序列。以前为了表示键盘上没有的字符，这是必不可少的一种方法。</p>
<p>三字符序列可以出现在任何地方，包括字符串、字符序列、注释和预处理指令。</p>
<p>下面列出了最常用的三字符序列：</p>
<table class="reference notranslate">
<tr><th>三字符组</th><th>替换</th></tr>
<tr><td>??=</td><td>#</td></tr>
<tr><td>??/</td><td>\</td></tr>
<tr><td>??'</td><td>^</td></tr>
<tr><td>??(</td><td>[</td></tr>
<tr><td>??)</td><td>]</td></tr>
<tr><td>??!</td><td>|</td></tr>
<tr><td>??&lt;</td><td>{</td></tr>
<tr><td>??&gt;</td><td>}</td></tr>
<tr><td>??-</td><td>~</td></tr>
</table>
<p>如果希望在源程序中有两个连续的问号，且不希望被预处理器替换，这种情况出现在字符常量、字符串字面值或者是程序注释中，可选办法是用字符串的自动连接："...?""?..."或者转义序列："...?\?..."。</p><p>
从Microsoft Visual C++ 2010版开始，该编译器默认不再自动替换三字符组。如果需要使用三字符组替换（如为了兼容古老的软件代码），需要设置编译器命令行选项/Zc:trigraphs</p><p>
g++仍默认支持三字符组，但会给出编译警告。</p>

<h2 class="tutheader">C++ 中的空格</h2>
<p>只包含空格的行，被称为空白行，可能带有注释，C++ 编译器会完全忽略它。</p>
<p>在 C++ 中，空格用于描述空白符、制表符、换行符和注释。空格分隔语句的各个部分，让编译器能识别语句中的某个元素（比如 int）在哪里结束，下一个元素在哪里开始。因此，在下面的语句中：</p>
<pre>
int age;
</pre>
<p>在这里，int 和 age 之间必须至少有一个空格字符（通常是一个空白符），这样编译器才能够区分它们。另一方面，在下面的语句中：</p>
<pre>
fruit = apples + oranges;   // 获取水果的总数
</pre>
<p>fruit 和 =，或者 = 和 apples 之间的空格字符不是必需的，但是为了增强可读性，您可以根据需要适当增加一些空格。</p>			<!-- 其他扩展 -->
						
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<div id="postcomments"  style="display:none;" >
	<ol class="commentlist">
		<li class="comment even thread-even depth-1" id="comment-11269"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;绝罗</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;419***405@qq.com</p></div><div id="runoobvote-id-11269" data-commid = "11269" class="upvotejs"><a class="upvote"></a> <span class="count">409</span></div></div><div class="comt-main" id="div-comment-11269"><p>现在大家买的C++教材可能会经常看到头文件是如下:
</p><pre>
#include &lt;iostream&gt;</pre><p>
然后用现在较新的软件则会提示：无法打开源文件"iostream.h"。</p>
<p>
这是因为现在新的软件都是采用标准 C 格式，而 iostream.h 是 VC6.0 使用的风格，也就是标准 C 格式还未出台之前的风格。
</p><p>
出现上述提示只需要进行如下修改:
</p><pre>
#include &lt;iostream&gt;
using namespace std;
</pre><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">绝罗</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;绝罗</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;419***405@qq.com</p></div></span>6年前 (2017-04-22)</div></div></li><!-- #comment-## -->
<li class="comment odd alt thread-odd thread-alt depth-1" id="comment-16906"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;赵斌</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;284***537@qq.com</p></div><div id="runoobvote-id-16906" data-commid = "16906" class="upvotejs"><a class="upvote"></a> <span class="count">196</span></div></div><div class="comt-main" id="div-comment-16906"><p>C/C++可以使用带有 <span class="marked">$</span> 的标识符：</p>

<pre>#include "stdio.h"

int main(){
        int _2a$=0x99;
        printf("%d\n",_2a$);
        return 0;
}</pre><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">赵斌</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;赵斌</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;284***537@qq.com</p></div></span>6年前 (2017-08-28)</div></div></li><!-- #comment-## -->
<li class="comment even thread-even depth-1" id="comment-17778"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;一条柴</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;332***496@qq.com</p></div><div id="runoobvote-id-17778" data-commid = "17778" class="upvotejs"><a class="upvote"></a> <span class="count">163</span></div></div><div class="comt-main" id="div-comment-17778"><p>补充上面 <span class="marked">$</span> 可以放在前面:</p>

<pre>
#include "stdio.h"

int main(){
    int $a=0x99;
    printf("%d\n",$a);
    return 0;
}
</pre>
<p>甚至是：</p>
<pre>#include "stdio.h"

int main(){
    int $8=0x99;
    printf("%d\n", $8);
    return 0;
}</pre><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">一条柴</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;一条柴</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;332***496@qq.com</p></div></span>6年前 (2017-09-19)</div></div></li><!-- #comment-## -->
<li class="comment odd alt thread-odd thread-alt depth-1" id="comment-22257"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;eric</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;gxy***@163.com</p><p><i class="fa fa-external-link" aria-hidden="true"></i> <a rel="nofollow" target="_blank" href="https://zhidao.baidu.com/question/24030007.html?fr=iks&amp;word=%CE%AA%CA%B2%C3%B4C%2B%2B%D6%D0%2C%D6%F7%BA%AF%CA%FDmain%C7%B0%B6%BC%D2%AA%BC%D3%B8%F6%C0%E0%D0%CD%D0%DE%CA%CE%B4%CA%2C%C0%FDint%2Cvoid.">&nbsp;&nbsp;参考地址</a></p></div><div id="runoobvote-id-22257" data-commid = "22257" class="upvotejs"><a class="upvote"></a> <span class="count">241</span></div></div><div class="comt-main" id="div-comment-22257"><p data-title=" C ++中 main 函数前面为什么要加上数据类型？"><strong>在 C++ 中 main 函数前面为什么要加上数据类型，比如： <span class="marked">int void</span> ?</strong></p>
<p><strong>main</strong> 函数的返回值是返回给主调进程，使主调进程得知被调用程序的运行结果。</p>
<p>标准规范中规定 <strong>main</strong> 函数的返回值为 <strong>int</strong>，一般约定返回 0 值时代表程序运行无错误，其它值均为错误号，但该约定并非强制。</p>
<p>如果程序的运行结果不需要返回给主调进程，或程序开发人员确认该状态并不重要，比如所有出错信息均在程序中有明确提示的情况下，可以不写 main 函数的返回值。在一些检查不是很严格的编译器中，比如 VC, VS 等，void 类型的 main 是允许的。不过在一些检查严格的编译器下，比如 g++, 则要求 main 函数的返回值必须为 int 型。</p>
<p>所以在编程时，区分程序运行结果并以 int 型返回，是一个良好的编程习惯。</p><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="../note/22257">eric</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;eric</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;gxy***@163.com</p><p><i class="fa fa-external-link" aria-hidden="true"></i> <a rel="nofollow" target="_blank" href="https://zhidao.baidu.com/question/24030007.html?fr=iks&amp;word=%CE%AA%CA%B2%C3%B4C%2B%2B%D6%D0%2C%D6%F7%BA%AF%CA%FDmain%C7%B0%B6%BC%D2%AA%BC%D3%B8%F6%C0%E0%D0%CD%D0%DE%CA%CE%B4%CA%2C%C0%FDint%2Cvoid.">&nbsp;&nbsp;参考地址</a></p></div></span>5年前 (2017-12-28)</div></div></li><!-- #comment-## -->
<li class="comment even thread-even depth-1" id="comment-26656"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;长风习习</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;118***7045@qq.com</p></div><div id="runoobvote-id-26656" data-commid = "26656" class="upvotejs"><a class="upvote"></a> <span class="count">135</span></div></div><div class="comt-main" id="div-comment-26656"><p data-title="C 语言 int main() 和 int main(void) 的区别？"><strong>C 语言 int main() 和 int main(void) 的区别？</strong></p>

<p><span class="marked">int main(void)</span> 指的是此函数的参数为空，不能传入参数，如果你传入参数，就会出错。</p>


<p><span class="marked">int main()</span> 表示可以传入参数。</p>

<pre>// 这样是正确的
int main()
{
  if (0) main(42);
}

// 这样会出错
int main(void)
{
  if (0) main(42);
}</pre>
<p>
在 C++ 中 <span class="marked">int main()</span> 和 <span class="marked">int main(void)</span> 是等效的，但在 C 中让括号空着代表编译器对是否接受参数保持沉默。在 C 语言中 main() 省略返回类型也就相当说明返回类型为 int 型，不过这种用法在 C++ 中逐渐被淘汰。虽然 void main（）在很多系统都适用，但他毕竟不是标准的，所以应该避免这种用法, 应该使用这种 <span class="marked">int main(void)</span> 的写法比较妥当。</p><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="../note/26656">长风习习</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;长风习习</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;118***7045@qq.com</p></div></span>5年前 (2018-04-25)</div></div></li><!-- #comment-## -->
<li class="comment odd alt thread-odd thread-alt depth-1" id="comment-28682"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;aaa</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;110***.com</p></div><div id="runoobvote-id-28682" data-commid = "28682" class="upvotejs"><a class="upvote"></a> <span class="count">83</span></div></div><div class="comt-main" id="div-comment-28682"><p data-title="C++ 标识符"><strong>标识符</strong></p>

<p><strong>定义：</strong>变量、符号常量、函数、数组、类型、文件、标签和其他各种用户定义的对象的名称。</p>
<ul><li>
第一个字符必须是字母或下划线
</li><li>
后续字符可以是字母、数字或下划线
</li><li>
标识符的有效长度不超过247字符
</li><li>
标识符不能和关键字相同
</li><li>
标识符区分大小写</li><li>
最好也不要和系统预定义标识符同名</li><li>
标识符命名要做到“见名知义”</li><li>
应该避免使用可能引起混淆的字母</li></ul>
<div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="../note/28682">aaa</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;aaa</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;110***.com</p></div></span>5年前 (2018-06-23)</div></div></li><!-- #comment-## -->
<li class="comment even thread-even depth-1" id="comment-46162"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;墨玉竹</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;932***556@qq.com</p></div><div id="runoobvote-id-46162" data-commid = "46162" class="upvotejs"><a class="upvote"></a> <span class="count">239</span></div></div><div class="comt-main" id="div-comment-46162"><p data-title>为什么要使用 <span class="marked">using namespace std;</span>  ?</p>

<p>有些名字容易冲突，所以会使用命名空间的方式进行区分，具体来说就是加个前缀。比如 C++ 标准库里面定义了 vector 容器，你自己也写了个 vector 类，这样名字就冲突了。于是标准库里的名字都加上 std:: 的前缀，你必须用 std::vector 来引用。同理，你自己的类也可以加个自定义的前缀。</p>

<p>但是经常写全名会很繁琐，所以在没有冲突的情况下你可以偷懒，写一句 using namespace std;，接下去的代码就可以不用写前缀直接写 vector 了。</p><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="../note/46162">墨玉竹</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;墨玉竹</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;932***556@qq.com</p></div></span>4年前 (2019-08-27)</div></div></li><!-- #comment-## -->
<li class="comment odd alt thread-odd thread-alt depth-1" id="comment-55804"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;Glory</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;171***7295@qq.com</p></div><div id="runoobvote-id-55804" data-commid = "55804" class="upvotejs"><a class="upvote"></a> <span class="count">266</span></div></div><div class="comt-main" id="div-comment-55804"><p>命名空间（namespace）的意思：</p><p>举个例子吧，我们的名字，有很多时候会重名，比如张三。</p><p>世界上可能有几十几百个张三。</p><p>我们的世界里当然可以用身份证号码来分啦，但在没有身份证的时代怎么分呢?</p><p>有办法的，比如，可以在名字前面加上地方名，如</p><p>广东的张三</p><p>上海的张三</p><p>同样的道理，我们在编程时，不同的程序员可能都会喜欢用同样的名字来表示相同的东西，如，大家可能都会用 dog 来表示狗类。好了，现在有好几位程序员都写了 dog 类，可能各自的功能不同或相同。就好比好几家人都养了条狗一样。好了，在程序调用时，编译器傻了，它不知道该拉哪家的狗出来响应呀!</p><p>怎么办？哦，让我们在狗前面加上 <strong>namespace(名字空间)</strong> 以示区分，好了，于是程序中就有了</p><p>张三 的 狗</p><p>李四 的 狗</p><p>啰嗦了一大堆，该说回来了</p><p>std：：</p><p>std 表示是 C++ 的标准命名空间，就是编译系统自带有的，按 C++ 标准定义好了的。</p><p>:: 书写格式</p><p>比如，在使用输出 <span class="marked">std::cout</span> 时,如果它达不到我们想要的效果，我们也可以自己定义一个名字空间。</p><p>取名 myspace，再在这个空间里写一个 cout 函数来实现。调用时，就成了 <span class="marked">myspace::cout</span>。</p><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">Glory</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;Glory</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;171***7295@qq.com</p></div></span>3年前 (2020-07-30)</div></div></li><!-- #comment-## -->
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